Relaxation oscillators capable of generating a very precise and stable oscillation frequency may be implemented using stable resistive and capacitive components together with a threshold device, such that the capacitance charges gradually and discharges rapidly. The resonant frequency of a conventional resistive-capacitive (RC) relaxation oscillator is derived from a time constant based on the resistive and capacitive components. In relaxation oscillator embodiments utilizing a comparator, the comparator contributes to overall non-zero propagation delay and adds to the period of the oscillator output. However, the propagation delay of a comparator may vary with temperature, supply voltage, input slew rate, input bias levels, parasitic capacitance, process corners, and other variables. This limitation complicates generation of a precise and stable oscillation frequency in a comparator-based relaxation oscillator.
One approach to achieving a very precise output frequency with a comparator-based relaxation oscillator is to make the comparator propagation delay negligibly small relative to the RC time constant. In addition to being difficult to implement, very fast comparators (with a proportionately small propagation delay) consume significant power. In addition, such very fast comparators typically require a very short channel (on the order of 0.18 microns) and high-speed complimentary metal-oxide-semiconductor (CMOS) technology.
Rather than minimizing the magnitude of the comparator propagation delay to generate a precise relaxation oscillator output frequency, an alternative is to minimize temperature and voltage variation of the comparator's propagation delay and/or use trimming technology to reduce process variation. However, this approach limits the uppermost oscillation frequency achievable and adds to manufacturing expense.
There is, therefore, a need in the art for improved implementation of comparator-based relaxation oscillators.